Aspergillus fumigatus galactomannan was found by ELISA or RIA in urine from 7 of 8 patients with invasive aspergillosis caused by that species but not in urine from controls or from 2 patients infected with A. flavus. The assay appears useful in diagnosis of this potentially fatal infection. Aspergillus fumigatus was found to produce an extracellular substance which inhibits the ability of normal human serum to opsonize fungal cells for ingestion by human monocytes. The substance impairs the alternative pathway of the complement cascade, causing reduced deposition of C3b on the fungal surface. A rapid enzymatic assay for the antifungal drug, flucytosine, was described. Creatinine iminohydrolase (EC 3.5.4.21) was used to quantitatively remove the amino group from flucytosine. Ammonia was then measured colorimetrically. Otherwise normal patients who are cured of cryptococcosis are specifically tolerant to immunization with cryptococcal polysaccharide. Recent work has shown that immunization of volunteers but not cured patients leads to circulating B cells with surface anticryptococcal antibody, indicating that tolerance in these patients occurs at an earlier stage of the humoral response. Immunoglobulin allotypes Gm 1,2,3,17;23,5,6,13,12, Kml and Km3 did not correlate with immune response of normal volunteers to cryptococcal or pneumococcal polysaccharide.